Making spheroidal powder



J. W. LYLE MAKING SPHEROIDL POWDER July -3,' .1962

med sept. 22'. 1958 mvemon JAMES WifL A-WORV@ cAs SHIELD cAs siucLD mia' 3,041,672 BIAKINC SllillROlDAL PWDIR .lames W. Lyle, Indianapolis,Intl., nssignor to Union Cm*- bide Corporation. u corporation of NewYork Filed Sept. 22, 1958, Ser. No. 762,296 6 Claims. (Cl. 15S-47.3)

. roidal shaped particles are usually preferred in contrast to irregularshaped particles obtained by mechanical grinding methods.

A novel process leas been developed, according to this invention, forproducing tine particles of sphcroidlal shape. 'Iltis process comprisesthe steps or' striking an arc between a stick electrode and a nozzleelectrode, said arc being wnll-stabilizcd at least along a pcrtion ofits length, introducing a consumable wire or rod into thc collimatedplasma etllucnt of such are, passing a gas stream along and coextensivewith such :ollimatcd plasma to shear off molten material from the moltensurface of the consumable wire whereby small droplets are formed andremoved from the are zone, and then solidifying and collecting theso-produccd finely-divided particlcs.

A wall-stabilized and constricted are of the tyre used in this inventionis described more fully in U.S. Patent No. 2,SO6.l24 and copendingapplication SN. 539.794, now Patent No. 2.858,4ll, dated October 28,i953.

The preferred operating condition for smallest particle size of productinvolves introducing the consumable wire or rod within the nozzleelectrode passage at the point of maximum arc construction and maximummomento.. of the arc plasma and coeztensivc gas stream. The gasstreamcmploycd should preferably be chemically inert to the moltenparticles so as to prevent reaction between the gas and particles inaddition to shielding the hot particles from atmospheric oxidation. VInaddition, for maximum melt-oli rates of the consumable wire and highestproduction rates for t'ine powder, the

consumable wire is preferably an electrode carrying the bulk of the arecurrent.

This process is especially useful for producing finely divided,spheroidal-shaped metal particles. Ilowerct', it may also be employedwith metal compounds, such as the refractory metal oxides, silicides,borides, nitrides and carbides, which are ditlicult if not impossible toproduce in line spheroidal form by prior art techniques.

These line particles are relatively uniform in shape and size. Theiractual size appears to be dependent upon arc power and gas momentum. Thesphereidal shape renders these products more free-flowing titan priorart irregular shaped particles obtained by mechanical grinding methods.'lltey are also useful for producing sintered porous bodies havingrelatively uniform pore size.

This process has the additional advantage of producing fine powders ofmaterial which has such a high d:- grce of duetility that it cannot bemechanically ground. Also, it can produce powders of refractory mamialswhich cannot he readily melted and treated in conventional shoz-towerprocesses.

In the drawings: n FIG. l is a fragmentary view m vertical cross-sectionof apparatus for making powder according to the invention; and

FIGS. Z-S are similar Yiews of other modifications.

y United Secties Patent @ffice Patented-July 3, 1962 electrode 10 andnozzle electrode Il. Electrical power is' supplied from source 12through leads t3 and 14. A gas stream passes down along electrode 10 andforces the arc 15 down into tite constrictcd portion 16 of the nozzlepassage 17 where the arc becomes wall-stabilized. The nozzle electrode11v is cooled below its melting point by passing cooling fluid, such a'swater, from'inlet 2S through passage 29 to outlet 30.

A consumable wire I8 is preferably introduced at or near tite point ofmaximum nozzle constriction and maximum arc plasma momentum in order toobtain line particle size product. This process is not limited, however,to wire introduction at this point. The wire may be introduced at otherpoints in thc nozzle or at the nozzle outlet as shown in the position ofwire 19 of FIG. 7.. Vrhen the wire 18 is introduced as shovm in vFIG. l,the nozzle passage extending beyond the point of wire entry tends toimprove results by focusing and controlling the position of the moltendroplet stream. It also directs the gas stream and maintains improvedshielding of the metal particles from atmospheric contamination. Adivergent l discharge passage 20 is conveniently employed primarily toreduce the possibility of nozzle plugging caused by deposits of moltenmetal particles. It also prevents the formation of an undesirable shockfront at the outlet which can occur with straight nozzles. A divergentanode passage is desirable in that it spreads the nozzle. electrode areaand reduces current density. Iltis helps reduce nozzle erosion at highcurrent levels. lt is preferred for the above reasons that the nouleoutict have an increased cross-sectional area as compared to the area ofthc` nozzle at the point of ire entry. Nozzle shapes other than thatshown might iso be used. 'Die molten particles 2t cntrained in the torchgas stream pass out of the apparatus and are solidi'tie and collected ina collector 22. This collector may, for e-:ampl:. contain a body ofwater into which thc molten particles are directed for soliditication.

In the apparatus modification shown in FIG. l the wire 1S can be inelectrical contact with nozzle electrode l1 and thus become anelectrode. When the wire extends into the nozzle passage the arc currentcan terminate at the wire instead of the nozzle. This increases wiremeltoff and increases production rates of tine particles.

Another modification of apparatus suitable for carrying out the presentnovel process is shown in FIG. 3. In this form the consumable wire 13 iselectrically insulated from nozzle electrode 1l by insulator 23. Thewire 18 can then be out of the electrical circuit, or in the preferredform can be almost electrically independent of the nozzle. The mainelectrical connections in the preferred form of this modification arefrom the power supply 12 through lead 13 to the stick electrode 10 andlead 2.4 to the consumable wire 18. The nozzle electrode 11 is connectedto the power supply through resistance 2S which tends to maintain thenozzle at a lower potenticn than that of the consumable wire. Thisapparatus can be operated at higher wire feed rates than that of FIG. l'because higher power levels can be maintained to the wire withoutdamage to the nozzle. This becomes important when wire feed rates ashigh as lbs/hr. are desired. A pilot arc is maintained between the stickelectrode and the nozzle electrode in ord-:r to effect startup oftheprocess and also to maintain an arc if wire feed ceases for any reason.The electrical contact from lead 28 to wire 18 is convenientlypositioned externally to the tcrch in order to increase resistanceheating along the wire and increase melt-off rates.

ln order to fully protect the molten particles from atmospheric.contamination prior to their soliditicatlon,

additional shielding gas can be introduced through shield process inoperation.

EXAMPLE I Production of Finely-Divided Srccl Particles An apparatus ofthe type shown in FIG. 1 was used consisting of a. lis-inch dia.throated tungsten stick cathode and a water cooled nozzle anode having athroat section r-inch da.. and ta-inch long. The nozzle passage beyondthe throat was s-inch long having a 30 divergent angle. The stickcathode was set hack J/a-inch from the throat section. An are of 150amperes and 85 volts was maintained between the stick cathode and nozzleanode while a gas mixture of ZOO c.f.h. argon and 13.5 c.f.h. hydrogenpassed along the tungsten cathode and out through the nozzle passage. AK-inch dia. steel wire (Linde No. 65 welding wire) was introduced at arate of about 7 1bs./ hr. to the ponle passage at a point adjacent tothe throat section. .e wire was in electrical contact to the nozzleanode ani thus l'ecame an effective anode as it projected into thenozzle passage. The molten particles from the wire were entrained in thegas stream and were collected in a container of water positioned aboutl-ft. from the torch oud-'L the resulting product particles were sulstaniah? an spheroidal in shape with particle size ranging from 5 to 200microns'in diameter. The average size based ondistribution was about l5microns in diameter.

EXAMPLE II Prcxfuction o] Finely-Divided Steel Particle:

The same equipment was used as described in Example I above. An are of.'50 ampercs :nfl 6d volts was maintained between the stick cathode andnozzle anode while a gas mixture of 200 c.f.h. argon and '13.5' c.f.h.hydrogen passed along the tungsten cathode and out through the nozzleessage. A V10-inch dia. steel wire was introduced EXAMPLE III Productionof Finely-Divided Tungsten Particles The same basic equipment describedin Example I above was used. In addition a brass tube 2-in. I.D. and4-ft. long nas attached to the torch outlet. The outlet end of this tubewas immersed in water. This is shown in FIG. 5. An arc of 125 amperesand 75 volts was maintained between the stick cathode and nozzle modeutile a gas mixture of 200 c.f.h. argon and 13.5 c.f.h. hydrogen passedthrough the torch. A IAG-inch dia. tungsten wire was introduced at arate of about 4.8 lbs./

tween thc stick cathode and nozzle anode while a' gas mirtturt` of 200lc.f.h. argonj-and l3.51 c.f.li. hydrogen passed along thetungstencathode' and out through the nozzle passage. A J/m-irlch da.sapphire rod was introduced to the nozzle Vpassage at la point adjacentto the throat section. The'rod was .ten-conducting and was thus not inthe electrical circuit. The molten particles lfrom the rod was entrainedin the gas stream and were collected in a container of water positionedabouthI-ft.

from the torch outlet. The resulting product particles weresubstantially all spherical in shape with an average particle s;ze ofabout 6 microns. t

EXAMPLE V Production of Finely-Divided Sapphire Par-ticle:

Table I I EFFECT OF AIIC POWER ON PAlt'IICI-.E SIZE v v A veraceVoltage, oils Current, Power, Particle Amps. kw. Stre- Mit-runs 310 1&6107 61. 1H) 10. 2 ISS 00 I 0 333 Dependency of particle size on gasmomentum is show-n below. The are power remained substantially constantand the win` feed rate was adjusted as required.

Table II EFFECT 0F OAS FLOW ON PARTICLE SIZE Gas Flow A retain PnrtlcloSlu, H yllmgcn, Argon, M tetons c.f.h. c.f.h.

It can be seen from the above tables that as the arc power and the gasmomentum through the torch, as inhr. to the nozzle passage adjacent tothe throat section.

The Wire was in electrical contact with the nozzle and thus beanie aneffective anode as it projected into the laltc. The molten particlesfrom the wire were en trained in the gas stream and were solidified andcollected l lb 'Hier located at the end of the protective tube. n POductparticles were substantially all spheroidal l? une with an averagediameter of 380 microns.

EXAP'IPL' IV Production o] Finely-Divided Sapphire Particles Eilmentdescribed in Example I above was und. #i me et N0 ampere: and 85 voltswas maintained bedicated by total gas flow through the torch, areincreased, the average particle size decreases.

The tine sphcroidal particles produced by the prent invention may beused as starting materials for miniature ball bearings or in powdermetallurgy for making sintered porous bodies of relatively uniform poresize. This novel process has the additional advantage of producing tinepowders of material, such as nickel-chromium alloy, which has such ahigh degree or ductility that it cannot be mechanically ground. Also, itcan produce powders of refractory materials, such as tungsten carbide,which cannot be readily melted and treated in conventional shot-towerprocesses.

What is claimed is:

t. Method of producing finely-divided spheroidal shaped particles ofmetals and metal compounds by striking a wall-stabilized are between astick electrode having an are constricting passage and a nozzleelectrode, in-

troducing an consumable wire or rod composed of said y metals and metalcompounds into the eollimated plasma such collima-ted ylasma through thenozzle passngeiof the formed and removed from the arc zone, and thensolidi-v [ying and collecting the so-produced finely-divided particles.

2. Method of producing nely-divided spheroidaishaped particles of metalsand metal compounds by striking a' wall-stabilized .virbetwcen a stickelectrode and a nozzle electrode havfng an arc eonstricting passage,introducing a consumable wire or rod composed of said metals and metalcompounds into the collmatcd plasma effluent of such arc, passing a gasstream coe'xtcnsive with such oollimated plasma through the nozzlepassage of the nozzle electrode to shear olf molten material from thetip of the consumable wire whereby small droplets are formed and removedfrom the arc zone, and then solidifying and collecting the so-produced`finely-divided particles,v in which the preferred operating conditionfor smallest particle size involves introducing the consumable metalwire at the point of maximum are constriction and maximum momentum ofthe arc plasma and coextensive gas stream.

3. Method as defined by claim 2, in which, for maximum melt-off rates ofthe consumable wire and highest production rates for fine powder, theconsumable wire is preferably an electrode carrying the bulk of the arccurrent.

4. Process as defined by claim 2, in which such are ellluent isconically expanded and thereby is focussed downwardly in the directionof a liquid in a container disposed thereunder for collecting suchsprny'of individual spheroids.

5. Process as defined by claim 4, in which such wire is laterally fedinto one side of such downwardly focussed eilluent so that suchparticles are sliearcd therefrom by such are ellf'ent which comprisesinert gas.

6. The method of producing very small spheroidalshaped particles ofmetals and metal compounds by striking an arc between a trstnonconsumable electrode and a second constricting nonconsumable nozzleelectrode having an expanding conical outlet, passing a gas stream' incontact with said electrodes and through said nozzle electrode toproduce a collimated plasma cflluent, introducing a consumable wire orrod of said metals and metal compounds into the collimated plasma at apoint of maximum are constriction and plasma momentum, whereby saidvn're is melted and disintegrated into small droplets and removed fromthe are zone bythe plasma stream., and then solidifying and collectingthe resulting spheroidal powder particles.

References Cited in the lc of this patent UNITED STATES PATENTS1,128,175 Morf Feb. 9, 1915 1,133,508 Seboop Mar. 30, 1915 2,189,387Wissler Feb. 6, 1940 2,269,528 Gallup Jan. 13, 1942y 2,768,279 Rava Oct.23, 1956 2,770,708 Briggs Nov. 13, 1956 2.795.819 Lezberg et al June 18,1957 2,806,124 Gage Sept. 1t), 1957

1. METHOD OF PRODUCING FINELY-DIVIDED SPHEROIDALSHAPED PARTICLES OFMETALS AND METAL COMPOUNDS BY STRIKING A WALL-STABILIZED ARC BETWEEN ASTICK ELECTRODE HAVING AN ARC CONSTRICTING PASSAGE AND A NOZZLEELECTRODE, INTRODUCING AN CONSUMABLE WIRE OR ROD COMPOSED OF SAID METALSAND METAL COMPOUNDS INTO THE COLLIMATED PLASMA EFFLUENT OF SUCH ARC,PASSING A GAS STREAM COEXTENSIVE WITH SUCH COLLIMATED PLASMA THROUGH THENOZZLE PASSAGE OF THE NOZZLE ELECTRODE TO SHEAR OFF MOLTEN MATERIAL FROMTHE TIP OF THE CONSUMABLE WIRE WHEREBY SMALL DROPLETS ARE FORMED ANDREMOVED FROM THE ARC ZONE, AND THEN SOLIDIFYING AND COLLECTING THESO-PRODUCED FINELY-DIVIDED PARTICLES.